The present invention relates to anhydrous non metal and metalloid halides, which have very important uses in various industrial applications. Thus, they are used as intermediates in the production of metals (e.g. titanium and zirconium from the corresponding tetrachlorides) and of pure oxides (e.g. TiO.sub.2 and SiO.sub.2 from the corresponding purified tetrachlorides). Moreover, many of these chlorides (e.g. aluminum chloride, ferric chloride, titanium chlorides) are very useful catalysts, and they also serve as starting materials for the insertion of such elements in organic compounds.
At present, some of these anhydrous non metal and metalloid halides are produced, at comparatively high temperatures, by reacting the metal oxide with a halogen, usually chlorine, and a reducing agent (mostly carbon, obtained from coke, pitch, etc.). Titanium tetrachloride, for instance, is made according to the following equation: EQU TiO.sub.2 +2Cl.sub.2 +C.fwdarw.TiCl.sub.4 +CO.sub.2 (+CO)(at about 800-1000.degree. C.)
Silicon tetrachloride is usually made in two stages via silicon carbide:
(1) SiO.sub.2 +3C.fwdarw.SiC+2CO (in an electric furnace at about 2200.degree. C.) PA0 (2) SiC+2Cl.sub.2 .fwdarw.SiCl.sub.4 +C (at about 400.degree. C.)
The use of the ever more expensive high-quality coke and energy-intensive chlorine is, of course, less appealing today than it used to be. It should also be observed at this point that the gaseous "by-products"--typically a mixture of CO.sub.2, CO and some phosgene diluted by much nitrogen--are next to useless and may actually provide an additional separation problem in the course of the purification of a volatile halide such as BCL.sub.3 (see Chun, U.S. Pat. No. 4,238,465).
It is also known that metal chlorides can be produced from their corresponding oxides by a process known as chloridization, i.e. the metathetic reaction of oxides with alkali chlorides such as sodium chloride, or with ammonium chloride, or with alkaline earth metal chlorides, such as calcium chloride and to a lesser extent with magnesium cloride. Thus, anhydrous stannic chloride is produced by the reaction of stannic oxides with magnesium chloride (G. S. Frents, Izvest. Akad. Nauk S.S.S.R., Otdel. Tekh. Nauk, 1948, 235-8; Chem. Abstr., 42,6690c (1948); EQU 2MgCl.sub.2 +SnO.sub.2 .fwdarw.SnCl.sub.4 +2MgO (at 700.degree.-800.degree. C.)
Copper, nickel, cobalt and manganese are extracted from ocean floor nodule ores, in which they are present mainly in the form of oxides, by chloridizing with mixtures of alkali and alkaline earth chlorides usually at 600.degree.-1000.degree. C. (Kane et al, U.S. Pat. No. 3,894,927). The chloridization method has not, to our best knowledge, been applied to those oxides that are considerably more stable, from the thermodynamic point of view, than the corresponding chlorides (as shown by comparing the free energy of formation of the two compounds). This class includes many oxides, typically of elements from groups IIIA-VA and IIIB-VIB of the periodic table (e.g. titanium dioxide, zirconium dioxide, aluminum oxide, boron oxide etc.) It is an object of this invention to prove the applicability of the chloridization method--mostly by magnesium chloride--in this part of the field.
In some chloridization reactions, silica was added in order to bind the basic oxide (such as sodium or calcium oxides) formed from the chloridizing reagent, see for example K. Sziklavary, Banyasz. Kohash, Lapok, Kohasz., 109, 177-80 (1976)--Chem. Abstr., 85, 146266t (1976); U. Kuxmann and F. Odor, Z. Erzbergbau Metalhuettenw., 19 (8), 388-97 (1966). However, to our best knowledge there is today no published research to be found concerning the possible applications of by-products (in the quoted examples--silicates or chlorosilicates of calcium) formed in such reactions.
It is this type of compound--especially such as contain magnesium oxide chemically combined with other oxides(s)--that constitutes the second object of our invention. In particular, the magnesium silicates--forsterite (Mg.sub.2 SiO.sub.4) and enstatite (MgSiO.sub.3), magnesium aluminum silicate or cordierite (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18) and magnesium aluminate or spinel (MgAl.sub.2 O.sub.4) are among the most important phases encountered in ceramic bodies, especially in refractories and in electrical ceramics. Other similar compounds, e.g. titanates of magnesium and magnesioferrite, are also components of some dielectric and ferrimagnetic materials. At present, the sources for the oxides of which the said magnesium compounds in the fired ceramic body consist are, usually:
(a) Natural minerals (such as serpentine, talc, olivine, kaolinite). Their main drawback is that only partial purification is possible, and as a rule a large part of the impurities are incorporated into the final product. (For a discussion of this problem with respect to forsterite manufacture and use and an example of partial purification of raw materials, see Aitcin, U.S. Pat. No. 4,287,167).
(b) Pure oxides and/or pure salts (such as carbonates or oxalates) which yield the oxide through thermal decomposition. The problem of the impurities is largely solved, but unfortunately the cost of the reactants limits the use.
(c) A ground mixture which consists essentially of the same crystalline phase desired in the final product, with or without additives. This starting material will often yield the products of the best quality, because of the improved microscopic uniformity of the solid, its smaller volume change during sintering and sometimes the higher sintering rate. (See, for example, Kostic & Momcilovic, in Advances in Ceramics Processings, Proceedings of the 3rd International Meeting on Modern Ceramic Technologies, Rimini, May 27-31, 1976--pp. 344-347). However, an additional step is of course required--the synthesis of the said crystalline phase, preferably by method (b)--which further raises the cost. This is mainly why pure forsterite, cordierite and enstatite are seldom used as starting materials in ceramic practice. This invention seeks to provide a remedy to this state of affairs.
The importance of pure reactants in ceramics should not be underestimated. Not so long ago, impurities in the raw materials were often treated as a "blessing in disguise" (indeed sometimes they still are), since they tend to widen the firing temperature range of ceramic bodies by lowering the lowest eutectic temperature in the system. With the continuously improving temperature control in firing ovens, the firing range factor has lost much of its importance. At the same time, the growing demand for specialized, high quality electrical ceramics underlines the shortcomings of bodies fabricated from impure raw materials. Likewise, owing to the increasing prices of refractories and to the increasing importance of good energy management, steel producers and other users have been growing increasingly more demanding with respect to quality and durability of refractories.